Laboratory tests to diagnose Legionella pneumonia are currently very insensitive, relying mainly on the culture of the organism, which may require up to 2 weeks, or on serologic conversion, which may take 2 to 4 weeks. In recent years, a urinary antigen assay has become commercially available: first as a radiometric assay, but more recently in an enzyme immunoassay format. We have evaluated this assay using control organisms and random patient specimens and have found it to be highly specific, detecting only Legionella pneumophila, serotype 1. This assay is a welcome addition to diagnostic tests for Legionella but may be insufficient because of its limited specificity, detecting only serotype 1. Recent published reports suggest that polymerase chain reaction (PCR) assay may provide a more sensitive technique for diagnosis. We have used a commercially available PCR assay for detection of Legionella from environmental samples as a starting point for developing a test for clinical specimens, particularly for sputum and bronchoalveolar lavage. The advantage of this procedure is its ability to detect almost all species of the genus Legionella, as well as to identify by specific probe the presence of L. pneumophila. Identification of other species would need further development. Another significant advantage is the inclusion of a simple dot-blot assay, which we found sensitive enough to detect less than 50 colony-forming units (using spiked specimens). We are in the process of screening all induced sputa and bronchoalveolar lavage specimens submitted for routine culture to determine the incidence of specimens that are positive by this assay. For all PCR positive assays, we will then look at patient history, urinary antigen assay, culture, and serologic conversion to assess the validity of the PCR results. Thus far, we have detected one positive induced sputum specimen that was validated by a significant serologic conversion despite its having a negative urinary assay.